The present invention relates to coatings containing fillers and fillers in filler-polymer compositions. The present invention further relates to ultraviolet (UV) light curable coatings, and cured coatings or films formed therefrom, containing filler-polymer compositions with low dielectric constant, high resistivity, and good optical density properties, and controlled electrical resistivity. The present invention further relates to black matrices containing filler-polymer compositions with low dielectric constant, high resistivity, and good optical density properties, and controlled electrical resistivity. The present invention further relates to black column spacers containing filler-polymer compositions with low dielectric constant, high resistivity, and good optical density properties, and controlled electrical resistivity. The present invention further relates to light shielding coating elements for LCD devices containing filler-polymer compositions with low dielectric constant, high resistivity, and good optical density properties, and controlled electrical resistivity. The present invention also relates to devices having these black matrices, black column spacers, and light shielding coating elements. The present invention further relates to methods of preparing and making these various materials and products.
Black matrix is a generic name for materials used in color displays to improve the contrast of an image by separating individual color pixels. Electric color display devices convert electric information into images. In liquid crystal displays (LCDs), the black matrix can be a thin film having high light-shielding capability and can be formed between the three color elements of a color filter. One conventional LCD device is a thin film transistor liquid crystal display (TFT-LCD). In LCD's using thin film transistors (TFT), the black matrix also can prevent the formation of photo-induced currents due to reflected light in the TFT. Color filter on array (COA) technology also has been developed in which a COA-TFT substrate of a LCD device is provided. Some developments in color filter on array technology are shown, for example, in U.S. Pat. Nos. 7,773,177; 7,439,090 B2; 7,436,462 B2; and 6,692,983 B1; and U.S. Patent Application Publication No. 2007/0262312 A1. Black matrixes, for example, have been patterned on a thin film transistor (TFT) array substrate of a color filter on array (COA)-TFT structure to define regions where red, green, and blue filter layers are formed to provide a color filter on the TFT array substrate. An ink jetting process also has been used in the manufacture of a color filter of an LCD. In one form of ink jetting process, a light-shielding layer such as a black matrix has been formed on a glass substrate component of a color filter structure, and the black matrix undergoes exposure and development processing to form a pixel area on the black matrix. Black matrix compositions also have been provided as photocurable compositions, such as photoresistive compositions.
A LCD device requires internal spacers to maintain a cell gap defined by a thickness of the liquid crystal layer. Ball spacers can be used to maintain the spacing. The ball spacers have a spherical form and can move in the gap they provide between two substrates. In a different design, column spacers can be used to maintain a constant gap for liquid crystal layer in LCD device. Unlike ball spacers, the column spacers are immobile. The layer of column spacer is generally preformed through a lithographic process similar to black matrix on the first or second substrate. The curable composition to make column spacers can be pigmented with particles such as carbon black. Such a spacer design is called a black column spacer. Since the column spacers have fixed position, they can be used as a light shielding element, particularly black column spacers.
In making filler-polymer compositions, there is a desire to have certain electrical resistivity, such as surface resistivity and/or volume resistivity, in the filler-polymer composition for various uses, such as black matrices, black column spacers or other light shielding coating elements in LCD. The desired range of electrical volume (or surface) resistivity depends on a particular application and, for example, can be in the range of from 101 to 1018 Ohm*cm. A typical volume resistivity value for some commercial polymers is in the range of from 1012 to 101 Ohm*cm. Conductive filler with good extinction coefficient, such as carbon black, is frequently added to lower electrical resistivity of particle-polymer composition and/or to provide good light shielding properties. A sharp change in resistivity of the composite happens when the concentration of carbon particles reaches a critical value at which continuous conductive paths are formed.
Black pigments such as carbon black have been used in polymer compositions to make resistive black matrices or other light shielding coating elements in LCD. However, typical systems may not be able to provide the desired balance of overall properties, such as with respect to the required light-shielding capabilities (e.g., an optical density (OD) of greater than 3 at 1 micron thickness) and resistivity. Modified pigments having attached organic groups have also been disclosed for use in a black matrix for color filters. For example, U.S. Patent Application Publication No. 2003-0129529 A1 relates, in part, to a black matrix prepared using a pigment having attached at least one polymeric group, wherein the polymeric group comprises at least one photopolymerizable group and at least one ionic or ionizable group. Also, U.S. Patent Application Publication No. 2002-0020318 A1 relates, in part, to a black matrix prepared using a pigment having attached at least one organic ionic group and at least one amphiphilic counterion. In addition, U.S. Patent Application Publication No. 2002-0011185 A1 relates, in part, to black matrix prepared using a photosensitive coating comprising solvent and pigment having attached at least one alkylene or alkyl group having 50-200 carbons.
The resistivity of compositions or composites can be affected by chemical fractionalization of the carbon black, for instance, by use of diazonium chemistry, where an alkyl-containing or aromatic-containing group is attached onto the carbon black. U.S. Published Patent Application No. 2006/0084751 A1 provides some examples of certain types of chemical functionalizations of the carbon surface via diazonium chemistry with non-polymeric organic groups. While this chemical fractionalization has been quite useful and an important advancement in filler-polymer compositions, the chemical fractionalization of carbon black can have a disadvantage in that the chemical organic groups attached onto the surface of the carbon black are sensitive to high temperatures. For instance, at temperatures above 150° C., the chemical groups attached onto the carbon black may be destroyed, which can lead to the loss of resistivity performance. Some filler-polymer compositions are made or are preferably made in high temperature processing or subjected to high temperatures in post-processing. In some processes for preparation of a black matrix, for instance, a coating film containing chemical-functionalized carbon black is exposed to an elevated temperature (e.g., a baking step for a coating). It would be helpful to have alternative solutions to controlling electrical resistivity in filler-polymer compositions for black matrices and other applications which are less sensitive to high temperature processing, and therefore, can provide more robust manufacturing process.
Further, it would also be desirable to have a filler which can permit control of electrical resistivity and dielectric constant in a polymer composition and provide good light shielding properties in various applications, such as in black matrices, black column spacers or other light shielding coating elements in LCD. In semiconductor manufacturing, a low-k dielectric is a material with small dielectric constant relative to silicone dioxide. The dielectric constant of SiO2, an insulating material commonly used in silicon chips, can be about 3.9. Carbon black itself typically has a significantly higher dielectric constant than silica. As indicated, the COA designs include the black matrix that can be directly coated on a thin film transistor (TFT). Such arrangement, although beneficial to improve aperture size and increase energy efficiency, may cause poor performance of a LCD device due to signal interference between the thin film transistor, the gate line or the data line. For example, U.S. Pat. No. 7,773,177 discusses the nature of parasitic capacitance induced in COA and relates the issue to the black matrices having high dielectric constant. The COA-TFT configuration necessitates materials for the black matrix that show low dielectric constant to prevent capacitive interference and signal delays. Additionally, the black matrix layer needs to be very resistive, but the optical density requirement for the layer is reduced since COA-TFT can utilize the black matrix with larger thickness than in the conventional design. Optical density in the range of 1-2 per micron layer thickness is targeted for COA-TFT configuration.
Further, as has been mentioned above, a LCD device requires internal spacers to maintain a constant cell gap for the liquid crystal layer. Since ball spacers are randomly distributed between the first and second substrates, the quality of an alignment layer may be lowered due to movement of the ball spacers. Moreover, a uniform cell gap may not be obtained in a large sized LCD device with ball spacers. Furthermore, since the ball spacers are elastic and do not remain at a fixed position, a severe ripple phenomenon may occur when the LCD device is touched. Thus, superior display quality cannot be obtained in the LCD device when a uniform cell gap is maintained using ball spacers. On the other hand, a uniform cell gap can be obtained using patterned spacers. In addition, the patterned spacers may be applied to an LCD device to form a small cell gap since the patterned spacers can be controlled precisely. Furthermore, since the patterned spacers are fixed, they may be easily applied to large sized LCD devices and the ripple phenomenon may be prevented when the LCD device is touched. When a black column spacer is used, it is important to maintain a high electrical resistivity and low dielectric constant for the spacer composition since the electric field can be distorted if more conductive and/or high dielectric materials are present near the electrodes and thin film transistors. This will lead to the poor quality operation of the LCD device. Generally, the dielectric constant of black column spacers should be less than 20, preferably less than 10, more preferably, less than 5.
Accordingly, there is a need to overcome the disadvantages mentioned above and to provide filler-polymer compositions useful for forming black matrices, black column spacers or other light shielding coating elements in LCD with good overall performance which can exhibit combinations of high resistivity, low dielectric constant, acceptable optical density properties, and controllable electrical properties in order to achieve desired resistivity and dielectric constant ranges, and particularly methods which are not dependent on the chemical fractionalization of conductive filler particles alone or at all.